This invention relates generally to protective coatings. More particularly, the present invention relates to discontinuous coatings of refractory metal particles such as abrasion-resistant, heat-resistant, or chemical resistant, or the like, materials, applied to surfaces that may be composed of metal, metallic alloys, and metal containing bases and the like.
Refractory metal coatings applied to base metals are particularly desirable to achieve a wear resistance that is not characteristic of the base metal. It is known that wear processes involving metal surfaces produce high temperatures at microscopic surface locations due to friction. These temperatures may often be beyond the melting point of particular base metals and therefore lead to undesirable surface changes, but are not above the melting points of the useful refractory metals. A base metal covered or protected by a refractory metal coating can therefore achieve a longer useful and economical life.
It is known in the art to apply metal coatings such as through metal diffusion processes for coating of other metals. Essentially, these processes utilize a powdered or otherwise finely divided impregnating material which may be a powdered metal, for example, and usually a source of volatile or vaporizable halogen-bearing substance. The various processes differ in many respects, but all the metal diffusing processes have one important factor in common, and that is, the temperature to which the potential metal coating and/or the metal surface to which it is to be applied, must be raised to a high degree. This temperature is usually in the range between 300.degree. C. and 1000.degree. C. in order to effect the diffusion of the coating metal upon the base.
While many of these processes are somewhat successful, they suffer from a number of disadvantages. Among these is the requirement of the extremely high temperature which necessarily will limit not only the type of material that may be used to form the coating, but also the surface of the article to be coated must be able to withstand exposure to such high temperature no matter how short the period of time.
None of these processes is adaptable for use with a significantly wide range of refractory materials, nor are any of the prior art metal diffusion processes capable of performing at any temperature but high temperature. Moreover, the limited applicability of the process to only those metal bases which are able to withstand the high temperature precludes a number of applications to metallic materials having relatively low melting temperatures. The application of refractory metals as protective coatings is also frequently difficult and uneconomical because a number of the refractory metals such as tungsten and molybdenum are prone to reaction with the atmosphere at high temperatures. Special equipment is therefore normally required to carry out processes involving these types of metals. For example, vacuum or inert atmosphere furnaces are frequently used.
There are other methods of applying a hard facing or other refractory coating to various base materials including flame-spraying processes, electroplating, dipping, or the like. Additionally, providing inserts of harder material in the softer to-be-protected material is an age-old technique still very prevalent in industry today.
While some of these methods may offer some protection for the article if the refractory coating can actually be applied successfully, they result in a particular disadvantage when the article to be coated is of a previously carefully shaped and proportioned size. The size and proportion may be critical but the added protective material will substantially alter the dimensions of the base article to an extent readily detectable by standard machine shop practice. The coated article may then require subsequent grinding or other processing to resurface and redimension the article. Obviously, this would not be acceptable to industry if the article were originally of a desired shape either in present use or adapted for immediate use, since any grinding or removal of the coating material to restore the original size and dimensions of the uncoated article would bring about a loss of the very protection that the refractory material is to provide.
Surface roughness is also a problem of concern. For instance, the roughness of the protective coating under the various prior art methods has been for some applications unacceptable even though the physical appearance of the surface deposit may seem to be smooth. While it may pass the inspection of the naked eye, or by touch, undesirable roughness is often found under high magnification or is evidenced by poor performance in service. This roughness may be in the form of protrusions of metal that have been found to have a tendency to flake and spall under abrasion, resulting in a loss of protection for the base metal.
Other disadvantages and drawbacks to prior art coatings and protective applications are the extreme brittleness of some coatings or materials which have been applied to achieve hardness, thus rendering the article to which such coating is applied impractical for use where any impact or bending is applied. One well-recognized fault of such coatings is their tendency to crack or spall from the substrate during service applications of the coated material.
Electroplating is another widely used coating procedure but inherent in this electrolytic process are a number of disadvantages other than the obvious requirements of careful controls and expensive electrical equipment. The electroplating process is basically limited to those metals that produce a substantial concentration of metal ions in an electrolyte bath. For some important metals (e.g., molybdenum, niobium, tantalum, titanium, tungsten and zirconium), the electrolyte is usually a molten salt bath to achieve this ion concentration, requiring great energy input not only in electric current but also in heat to maintain a molten condition. Other metals are limited by reason of their position in the electromotive series. Further, many practical applications necessitate the use of an electrolyte having a good throwing power in order to plate recessed or remote surfaces. Another major drawback associated with the electrodeposition of refractory materials from aqueous solutions is the low electrode efficiency which can be as small as a few percent for certain of the refractory metals. Such low efficiencies make the plating of refractory materials from aqueous solutions uneconomical.
The electroplated product is inherently a continuous coating but one that is not of uniform thickness due to the variations in current density produced by the shape and form of the plated article. In spite of the apparent continuity, the non-uniformity of the coating creates undesirable dimensional changes in the finished product, and in many cases generates residual stresses in the coating which, directly or indirectly, lead to cracking or spalling of the coating.
Small parts pose a particular problem for any electroplating process since each part must be electrically connected to a source of electrical current. To connect a great number of small knives, textile machine travelers or other small parts separately is at least an uneconomical task and even if performed the connection will leave an unplated area of the part. The alternative method of barrel plating small parts requires further expenditure for specialized equipment and has inherent limitations involving: non-uniform coating thickness, part shape limitations, and deleterious effects of part rusting.